Study On The Preparation And Properties Of Transparent Magnesium Hydroxide Nanodispersion And Its Nanocomposites

With the increasing fire accidents caused by the combustion of polymer materials, the flame retardancy of the polymers has attracted considerable attentions in recent years. As a green flame retardant, magnesium hydroxide (MH) has been widely used in flame-retardant polymer material research owing to its advantages such as halogen-free, smoke suppression, non-toxic and so on. However, the size of most MH particles used as flame retardant is larger than micro-size. A high loading level of MH is often needed to reach the satisfactory flame retardant effect, leading to severe deterioration of the processability and mechanical performance of the polymer. Nano-sized MH particles tend to aggregate with high surface energy, which limits the applications of MH in many fields. Therefore, achieving nanoscale dispersion of MH particles is the key to prepare polymer/MH nanocomposites with high performance. In this work, for the terminal applications of polymer materials, transparent MH nanodispersions in different liquid medium were synthesized via high-gravity technology. Based on the MH nanodispersions, polymer/MH nanocomposites with excellent transparency, mechanical and flame-retardant properties were prepared by different routes, including in-situ polymerization, solution blending and/or melt extrusion, by which the nanoscale dispersion of MH particles in the polymer matrix can be realized. The main contents are as follows:1. With MgCl2·H2O and NaOH as raw materials, transparent MH nanodispersions in different liquid medium (methanol, ethanol, ethylene glycol, toluene and water) were synthesized by high-gravity technology combined with surface modification. The MH nanodispersions were successfully manufactured in large scale. The effects of the preparation and modification condition on the dispersibility of MH nanoparticles in liquid medium were studied. The optimal condition is that the rotating speed of the high-gravity rotating packed bed is 2000 rpm, the feeding rate is 400 mL/min, the concentration of NaOH is 0.6 mol/L, the amount of the modifying agent of MH nanodispersion in ethanol is 10 wt%, the modifying temperature is 70℃, the modifying time is 3h, and so on. The MH nanodispersions have good transparency and stability. The lamellar MH nanoparticles with the size of 40～90 nm can be well-dispersed in liquid medium. Furthermore, the MH nanodispersion has better transparency by high-gravity technology compared with the traditional stirring method, and the size of MH nanoparticles is smaller with narrower size distribution, which is conducive to preparation of transparent nanodispersion with high solid content.2. PET/MH nanocomposite was prepared by the MH nanodispersion in ethylene glycol and the depolymerization product of PET via in-situ polymerization. The DSC data show that the temperature of crystallization peak of PET/MH nanocomposite increases from 189.1℃ to 195.8℃, indicating that the addition of MH nanoparticles significantly improve the crystallization of PET material. The residue in TGA test of PET/MH nanocomposite with 1 wt% MH prepared by MH nanodispersion is improved by 1.7% and 4.9% compared to pure PET and PET/MH nanocomposite prepared by MH powder, respectively. Moreover, the anti-dripping of the PET/MH nanocomposite has been significantly improved.3. Transparent PP/MH nanocomposite was successfully prepared with MH nanodispersion in toluene and PP by solution blending combined with melt extrusion. With this method, the difficulty of dispersing nanoparticles uniformly into polymer matrix during the melt blending process can be solved. The PP/MH nanocomposite has excellent transparency, thermal stability, flame retardancy and smoke suppression. The limiting oxygen index (LOI) of PP/MH nanocomposite with 100 phr MH is improved from 19.3 for pure PP to 29.7. The peak of heat release rate (PHRR), total heat released (THR) and total smoke rate (TSR) reduce by 71.6%,25.5%,44.6% compared to pure PP. Compared with the PP/MH composites prepared by nanosized MH powder (PP/np-MH) or microsized MH powder (PP/mp-MH), PP/MH composite prepared by MH nanodispersion (PP/nd-MH) has more excellent mechanical and flame-retardant properties. For example, the tensile strength of PP/nd-MH is improved by 34% and 31% compared to the PP/mp-MH and PP/np-MH. Importantly, a smooth and compact residue layer for the PP/nd-MH is formed during combustion, which yields a barrier for the transfer of heat and oxygen. The TEM image shows the MH nanoparticles in the PP/nd-MH can be well-dispersed in the matrix and keep their original morphology.4. Transparent PVA/MH nanocomposite film was prepared with MH nanodispersion in water and PVA by solution blending method. TEM images show that the MH nanoparticles disperse well in the PVA matrix at a nanoscale level, which benefits for the preparation of nanocomposite film with high transparency. The visible light transmittance of PVA/MH nanocomposite film can reach above 85% even at high MH content of 50 phr. The LOI of PVA/MH nanocomposite with 50 phr MH is improved by 45% compared to pure PVA, and the UL-94 can reach to V-0 level. The PHRR) in the microscale combustion calorimetry significantly reduces from 453 W/g for pure PVA to 332 W/g. These results indicate that the addition of MH nanoparticles can significantly improve the flame retardancy of PVA. By contrast, the PVA/MH nanocomposite filled with unmodified MH exhibits poor flame retardancy, and is opaque at high MH contents.